Previous research has used masking techniques to characterize both the spatial frequency and orientation tuning of mechanisms involved in human spatial vision. This proposal would use this information as a basis for investigating the problems of phase and position discrimination. The projects planned are designed to test the hypothesis that spatial phase discrimination is a special case of position discrimination. As virtually all previous research on phase discrimination has used cosine gratings, it has been impossible to produce phase shifts larger than 180 deg. By using spatially localized patterns with a 1.0 octave bandwidth, however, the proposed experiments will be able to measure position discrimination thresholds corresponding to arbitrarily large phase shifts. Major experiments will measure position discrimination with these patterns as a function of component spatial frequencies, contrasts, and temporal waveforms. Measurements will also be made at an eccentricity of 8.0 deg. for comparison with the foveal data. In addition, position measurements will be made in collaboration with Dr. Joel Pokorny on the chromatically isolated blue cone system, which has an inter-cone spacing on the order of 10.0 min. of arc. This will determine how a greatly reduced rate of spatial sampling affects position discrimination. Finally, these measurements will be repeated with amblyopes in collaboration with Dr. Marilyn Mets. This should provide the first measurements of just how poor position or phase discrimination really is in these subjects, and it will test the hypothesis that amblyopic phase discrimination deficits result from spatial undersampling.